Method of preparing yeast extract containing hydrolyzed non-yeast protein with yeast autolytic enzymes

ABSTRACT

Non-yeast protein is mixed with yeast, and enzymatic hydrolysis is carried out with yeast autolytic enzymes and optionally added exogenous enzymes to produce a yeast extract containing hydrolyzed non-yeast protein. More specifically, a mixture of yeast cells and 5 to 50% of cereal and/or animal non-yeast protein source is maintained at 40° to 50° C. for 5 to 15 hours and then at 55° to 65° C. for 1 to 5 hours to allow enzymatic hydrolysis to produce a water-soluble fraction. The water-soluble fraction is separated and concentrated. The non-yeast protein source may be one or more of maize gluten, corn gluten, wheat gluten, soya bean meal, whey solids, dried red blood, oat bran and wheat bran. By appropriate selection of the non-yeast protein, a yeast extract with specific flavor can be produced for use as a taste additive in the food industry.

The present invention relates to a process for producing yeast extractscontaining a proportion of hydrolysed non-yeast protein and the productof these processes.

Yeast extract is a nutritious palatable paste prepared from brewer's orbaker's yeast by autolysis. This comprises the self-digestion of theyeast cells principally through yeast proteolytic enzyme activity sothat proteinaceous soluble solids can be recovered. These soluble solidsare typically further concentrated to form a paste.

In order to maximise the yield of soluble solids for commercialproduction, yeast, such as brewer's yeast is conventionally diluted withwater to a specified solids content before autolysis. Salt may also beadded to the slurry to aid cell membrane rupture and exercise a degreeof control over microbial flora. The yeast proteins are solubilized andhydrolysed during the autolysis process. Although the natural yeastenzymes may be sufficient to carry out the hydrolysis, the activity ofthe yeast proteolytic system can be augmented, if desired, by theaddition of exogenous enzymes.

On completion of the autolytic process, the soluble fraction isharvested and concentrated by a series of evaporation steps to give atypical standard yeast extract. The autolytic process typicallysolubilises around 62% of the starting total yeast solids and yields amaximum of 80% of the yeast's original protein content.

The present invention is based on the surprising discovery that theyield of such an autolytic process can be improved and that,additionally, a range of novel flavors can be obtained by carrying outthe hydrolysis on a mixture of yeast and non-yeast protein.

According to the present invention there is provided a method of makingyeast extracts comprising subjecting an aqueous slurry containing amixture of yeast protein and non-yeast protein to an at least partialenzymatic hydrolysis to form a water-soluble fraction and recovering thewater-soluble fraction.

According to a preferred embodiment, the present invention comprises thesteps of:

(i) forming a slurry comprising yeast solids and water;

(ii) adding from 5 to 50% by weight based on yeast and non-yeast solidsof a non-yeast protein source to the slurry to form a mixture:

(iii) maintaining the mixture at a temperature of from 40° to 65° C. forfrom 6 to 20 hours;

(iv) separating the water-soluble fraction; and

(v) concentrating the water-soluble fraction.

The yeast protein is in the form of yeast cells and particularlypreferred sources are brewer's yeast and baker's yeast. These may beused in any form (such as dried yeast) but a particularly preferredsource is brewer's yeast in the form of a slurry obtained directly fromthe brewer.

Appropriate sources of non-yeast protein for use in the process of theinvention may include sources of cereal proteins and sources of animalproteins. Preferred non-yeast protein sources include maize gluten, corngluten, wheat gluten, soya bean meal, whey solids, soup stock and driedred blood. Additional protein sources such as oat bran and wheat branmay also be added. These non-yeast protein sources may be used directlyin the hydrolysis mixture or may be further treated, by purification orprotein extraction for example, before being used in the autolysismixture. The non-yeast protein sources may be used individually or asmixtures of two or more such sources. Particularly preferred non-yeastprotein sources for use in the present invention are soya bean meal andwhey solids which have been found to impart surprisingly good flavors tothe yeast extract products and also to be the most cost effectivealternatives.

The process of the present invention results in a yield of solublesolids which is greater than can be accounted for by the yeast alone.This appears to be due to the ability of the natural proteolytic enzymescontained in the yeast to act on the non-yeast protein to solubilise andhydrolyse it. Not only does the use of added non-yeast protein enablethe yield of normal autolysis procedures to be increased but the processmay also be used to maintain production levels when other yeast sourcesare in short supply by using the non-yeast protein as a yeast proteinextender. This is particularly useful when the source of yeast isbrewer's yeast and this is in short supply.

By selecting appropriate non-yeast proteins, the flavor of the yeastextract may be modified to impart novel flavors to the yeast extract.Such flavors are useful in altering the taste of the yeast extractitself but may also have wide application in the food industry as tasteadditives, especially where ingredients of a particular character arerequired. The surprising novel and interesting flavors of the yeastextracts produced in the present invention are believed to be due tosome form of synergistic effect resulting from the enzymaticco-hydrolysis of a mixture of yeast protein and non-yeast protein. It isbelieved that this is due, at least in part, to the action of yeastenzymes on both the non-yeast protein source and the yeast proteinsource. This is supported by the discovery that a mixture of standardyeast extract with an extract of hydrolysed soya bean meal was found toproduce flavor characteristics quite different from those of the yeastextracts of the present invention.

The present invention is based on the use of yeast proteases in theirnatural environment and at their natural relative concentrations tobreak down non-yeast protein. Yeast proteases become elevated when theyeast cell is starved and begins to digest its own protein. During theprocess, natural inhibitors to the major yeast proteases are digested ina sequential manner thus gradually releasing more and more proteolyticactivity. A cell-free system equivalent to this sequence would beextremely difficult to prepare. Yeast autolysis is thus regarded as anatural process and the present invention is based on the unexpectedfinding that a proportion of non-yeast protein can simultaneously behydrolysed without reducing the yield and surprisingly imparting noveland interesting flavors to the yeast extract product.

Non-yeast proteins can be conventionally digested to varying extentsusing one or more commercially available protease preparations. Thesecommercially available protease preparations are isolated, purified andconcentrated from microbial sources and usually have one principleactivity (e.g. NEUTRASE™ which is a serine protease). The nature of theproduct of such hydrolysis processes, however, is likely to be verydifferent from the product of yeast enzyme action, both in terms ofmolecular weight distribution and flavor. One unique aspect of thepresent invention using the yeast system lies in the wide range ofenzymes which are available for the hydrolysis of non-yeast proteini.e., proteases, amino peptidases and carboxypeptidases and which arenaturally designed to optimize yeast protein solubilization. There is nocommercial equivalent to this enzyme system. Such a sophisticated systemmight have been expected to be active against yeast substrates but notagainst non-yeast proteins. The finding that the yields were maintained(even increased) and that the resulting yeast extracts possess novel andinteresting flavors was thus unexpected.

The hydrolysis or autolysis processes of the invention include, amongstother chemical changes, the cleavage of protein chains to form smallerpeptides or amino acids. The hydrolysed mixture thus contains a mixtureof amino acid-based material from free amino acids through peptideoligomers to longer chain proteins. The mixtures may also contain othermaterial derived from the original starting mixture and this may includea variety of hydrolysed or unchanged material.

The process of the present invention costs approximately the same as aconventional yeast extract process since the extra yield compensates forthe additional cost of the non-yeast protein.

The enzymatic hydrolysis process of the present invention may be carriedout autolytically by maintaining the process conditions such that theyeast proteolytic enzymes effect the hydrolysis. Alternatively,exogenous enzymes may be added to the mixture to augment the activity ofthe yeast proteolytic enzymes. Suitable exogenous enzymes include plantproteases and bacterial proteases. Preferred proteases include papainand the bacterial protease made by submerged fermentation of a selectedstrain of Bacillus Subtilis and sold by Novo Nordisk Bioindustries UKLimited under the trade mark NEUTRASE™. The exogenous enzymes may beadded individually or as a mixture of different enzymes. The enzyme orenzymes may be added in varying amounts but are preferably added in anamount of from 0.003 to 0.02% by weight of the total hydrolysis mixtureat around 12% yeast solids.

Typically, the non-yeast protein is used in the present invention in anamount such that the mixture of yeast and non-yeast protein, beforehydrolysis, contains from 5 to 50% by weight non-yeast solids based onthe total weight of yeast and non-yeast solids. If the non-yeast solidscontent is below 5% of the total yeast and non-yeast solids, it has beenfound that there is insufficient modification of the flavor of the finalyeast extract product. On the other hand, if the amount of non-yeastsolids exceeds 50% of the total yeast and non-yeast solids, the yeastenzymes tend to be too dilute to effect sufficient hydrolysis thusresulting in low yields of soluble solids. Preferably the mixturescontain from 10 to 45% by weight non-yeast solids based on the totalweight of yeast and non-yeast solids, more preferably 15 to 40% and mostpreferably 30 to 35% (i.e. a ratio of yeast solids to non-yeast solidsof about 2:1).

The hydrolysis process may, optionally, be carried out in the presenceof sodium chloride typically in an amount of up to 1% by weight of totalhydrolysis mixture. This aids in the plasmolysis of the yeast cells andinhibits the growth of microbial flora.

Although the process of the present invention may be carried out undervarious conditions well-known to those skilled in the art, a typicalprocess sequence is set forth below.

An aqueous slurry containing yeast is produced by the addition of waterto pressed yeast or by dilution of a 14 to 18% yeast solids slurrytypically obtained from the brewery to give a 10 to 14% total solidsslurry, preferably about 12% total solids. Pressed brewer's or baker'syeast typically contains from about 22% to about 35% solids and may bemixed with water in mixing vessels to produce the desired slurrycomposition. To this mixture is added salt in an amount not greater than1.0% by weight of the total mixture and, optionally, one or moreexogenous enzymes. This mixture is maintained under conditions whichoptimize the activity of at least some of the enzymes present. Theseconditions generally involve incubation of the mixture at a temperatureof from 40° to 65° C. for from 6 to 20 hours. Preferred conditions forcarrying out the hydrolysis step involve maintaining the mixture underthe following conditions, sequentially:

Stage 1 : 40°-50° C. for 5 to 15 hours, particularly around 47° C. forabout 10 hours;

Stage 2 : 55°-65° C. for 1 to 5 hours, particularly around 60° C. forabout 2 hours;

Stage 3: at a temperature and for a length of time to pasteurise themixture and denature the enzyme, particularly around 90° C. for about 1hour.

It is believed that Stage 1 provides the optimum conditions for onegroup of yeast proteases and that Stage 2 provides the optimumconditions for a second group of yeast proteases. It is clear that theseconditions may vary depending on whether exogenous enzymes are added tothe mixture but the consequent alteration to the conditions would bewell-known to those skilled in the art.

After hydrolysis of the mixture has been carried out, the fractioncontaining soluble solids is separated from the crude hydrolysismixture. The separation can be achieved by conventional methods forseparating soluble solids from crude yeast hydrolysate mixtures in theproduction of standard yeast extracts by autolysis. Hence, in thepresent invention the soluble fraction may be obtained by centrifugalseparation and filtration. The resulting soluble fraction from thehydrolysis process can be concentrated by evaporation to about 40 to 50%total solids and the concentrate used as a base for process flavoringagents usually produced by controlled Maillard reactions. The solublefraction from the hydrolysis process may, alternatively, be concentratedby evaporation to form pastes containing from 55 to 80% total solids,preferably from 70 to 80% (e.g. around 75%). Salt may optionally beadded before the evaporation steps as is conventional in standard yeastextract processes.

The present invention thus provides yeast extract products having bothnovel and interesting flavors and a method for their production withoutreducing the yield of the standard autolysis process. As a result of thenovel flavors possessed by these yeast extract products, there are manypotential applications for the products where standard yeast extractwould not achieve the same effect.

EXAMPLE 1

1200 g of brewer's yeast slurry containing 12% solids, 1% salt and0.0094% papain by weight of total mixture was prepared and 72 g soyabean meal added. The whole was then autolysed with stirring at 47° C.for 10 hours followed by 60° C. for 2 hours followed by 90° C. for 1hour.

The soluble solids fraction was separated and evaporated and the finalextract prepared in the manner conventionally used for yeast extract.

The results were as follows:

    ______________________________________                                                                   Raw Material                                              Raw Material                                                                           Recovered  Cost/100 g                                                Cost     Solubles   Recovered Solubles                                 ______________________________________                                        Control   0.028     65.7 g      0.0426                                        (1200 g yeast                                                                 slurry)                                                                       + Soya bean                                                                             0.045     91.1 g      0.0494                                        meal (1200 g        (+39%)                                                    yeast slurry                                                                  +72 g Soya                                                                    bean meal)                                                                    ______________________________________                                    

EXAMPLE 2

Details as for Example 1 except that whey solids are used instead ofsoya bean meal.

The results were as follows:

    ______________________________________                                                                   Raw                                                                           Material                                                   Raw Material                                                                           Recovered Cost/100 g                                                 Cost     Solubles  Recovered Solubles                                 ______________________________________                                        Control    0.028     65.7 g     0.0426                                        (1200 g yeast                                                                 slurry)                                                                       + Whey Solids                                                                            0.046     98.2 g     0.0468                                        (1200 yeast          (+49%)                                                   slurry + 72 g                                                                 whey solids)                                                                  ______________________________________                                    

Thus, Examples 1 and 2 both demonstrate that increased raw material costis compensated for by a significant increase in recovered soluble.

In both cases, final extracts of the cohydrolysates were described byexpert assessors as having interesting novel flavors not usuallyassociated with brewers yeast extract.

EXAMPLE 3

1500 g of brewers yeast slurry containing 12% yeast solids and 1% saltby weight of total mixture was prepared and 60 g maize gluten granuleswere added. Papain was added and the mixture autolysed with stirring at47° C. for 10 hours followed by 60° C. for 2 hours followed by 90° C.for 1 hour. 10 The soluble solids fraction was separated and evaporatedand the final extract prepared in the manner conventionally used foryeast extract, adjusting the salt level to about 12% before the finalevaporation.

We claim:
 1. A method of making yeast extract containing hydrolyzed non-yeast protein, comprising the steps of:(i) forming a slurry comprising 10 to 14% yeast cell solids and water; (ii) adding to the slurry a non-yeast protein source selected from the group consisting of a cereal protein source, an animal protein source and mixture thereof in an amount of from 5 to 50% by weight based on yeast and non-yeast protein source solids to form a mixture; (iii) maintaining the mixture at a temperature of from 40° to 50° C. for 5 to 15 hours, and then at a temperature of from 55° to 65° C. for a period of from 1 to 5 hours to allow enzymatic hydrolysis to proceed to form a water-soluble fraction; (iv) separating the water-soluble fraction; and (v) concentrating the water-soluble fraction.
 2. The method as claimed in claim 1, wherein the hydrolysis is autolytic without added exogenous enzymes.
 3. The method as claimed in claim 1, wherein exogenous enzymes are added to the mixture.
 4. The method as claimed in claim 1, wherein the source of non-yeast protein is selected from the group consisting of maize gluten, corn gluten, wheat gluten, soya bean meal, whey solids, dried red blood, oat bran, wheat bran and mixtures thereof.
 5. The method as claimed in claim 1, wherein the mixture contains up to 1% by weight salt.
 6. The method as claimed in claims 1, wherein the water-soluble fraction is pasteurized.
 7. The method as claimed in claims 1, wherein the water-soluble fraction is concentrated by evaporation to from 40 to 50% total solids.
 8. The method as claimed in claim 1, wherein the water-soluble fraction is concentrated by evaporation to form a paste comprising from 55 to 80% total solids.
 9. The yeast extract containing hydrolyzed non-yeast protein produced by the method of claim
 1. 